The invention is relates to a spring strut with a work cylinder for supporting a vehicle chassis, as defined hereinafter. A work cylinder is already known in which to distinguish between an inward stroke and an outward stroke, a measuring system with two Hall generators or two Hall sensors is provided. In this work cylinder, the two Hall generators or sensors are disposed 90.degree. apart from one another.
The work cylinder includes a piston rod and a cylindrical tube, and there is a measuring segment on the cylindrical tube. The Hall sensors are disposed on the piston rod, and the measuring segment takes the form of a trapezoidal thread. By moving the sensors along the measuring segment, one signal per sensor is obtained, from which an electronic system can ascertain the direction of the relative motion.
The electronic system, the two sensors and the trapezoidal measuring segment entail a relatively major engineering effort and make themselves available as a high production cost. The electronics are not exactly simple, and so a low failure rate cannot actually be expected.